Rattling good fit !

[tim wood]

I’m going through the process of bleeding my brakes to get a firm pedal. ( not another bleeding thread I hear you groan). At the moment it’s a second pump that gets the pedal to the top where it should be.
Having bled until I’m blue in the face ( and language) I’ve started to look for more reasons why this problem exists.
On the rear brakes one pad runs true and close to the disc as it should be on the other pad , closest to the diff the pad has a definite clearance to the disc. What used to be called a “rattling good fit”.
This is the same both for left and right side of the car. The calipers are series 2 girling and overhauled. I can see that the discs aren’t central to the calipers.

Any thoughts, I’m loath to strip the IRS down

[mgcjag]

Uh....so you didnt centralise the caliper to the disc .....im just thinking out loud here.....how about a shim between back of brake pad and piston.....i know some cars have them as anti rattle shim...although im not sure here...becaus as the pads wear the pistons should take up the slack.....so you loose fit ones should just be like a worn pad.....do you have a firm pedal or does it need pumping....your "second pump"..doed sound like air in system though.....Steve

[ralphr1780]

Tim, obviously you need shims for the calipers to center the discs.

[tim wood]

Thanks both,
Steve - yes I’m thinking of using a shim twixt caliper and pad. This gap clearly shouldn’t exist .
My thoughts are that I’ll remove this obvious mechanical issue then that will determine if it is still a bleeding problem.


Ralph - yes thinking back now I had the IRS totally dismantled and it would have been better to do the job right first time !!

Hey Ho!

Tim

[mgcjag]

Hi Tim....the piston should push out to take up any gap.....and when pedal released not go back to its original loose position....so dont think you need fit a shim...is the loose piston actually pushing out when you press tne pedal....Steve

[tim wood]

Hi Steve,
On looking at the disc it’s clear where the pad has been in contact and the pad has been in use. It’s almost as though it’s pad knock back although I doubt it is.
Quite a conundrum. Something seems to be odd with the caliper but I’m so reluctant to drop the IRS.
If I have to at some point then it will come out easy.

I going to shim the pad and if that cures the issue until end of season then maybe I’ll investigate more.

If using a shim cures the pedal issue then I’m closer. If not then it’s back to further bleeding I think.

Tim

[tim wood]

Hi Steve,
On looking at the disc it’s clear where the pad has been in contact and the pad has been in use. It’s almost as though it’s pad knock back although I doubt it is.

I see that Dennis Welch sell a spring that fits inside the piston to make it’s kept close to the pad/ disc but this is probably for racers - which I’m not !!
Quite a conundrum. Something seems to be odd with the caliper but I’m so reluctant to drop the IRS.
If I have to at some point then it will come out easy.

I going to shim the pad and if that cures the issue until end of season then maybe I’ll investigate more.

If using a shim cures the pedal issue then I’m closer. If not then it’s back to further bleeding I think.

Tim

[Jack the lad]

You need to attach a dial test indicator to the diff. case with its stylus resting upon the face of the disc and then revolve the wheel by hand to see it the disc is running out of true. If it is then this is the problem, often caused by dirt/rust trapped between your disc and the output shaft carrier, or a badly made new disc or an abused (warped) old disc. Pedal needing two pumps is most likely air in system.

[tim wood]

Time for an update on this.
With the car supported on decent axle stands under the hubs I can quite clearly see this small gap between the pad and disc . It’s present on both left and right side of the car but greater on the right.

Without any pressure on the pedal the outer pad runs very close to the disc as you would expect whilst the inner pad has the “ rattling good fit”. With the pedal depressed then both pads grip the disc as you would expect and this small gap disappears. Once the pedal is released then the caliper piston retracts and the gap reappears!

In many years of buggering about with e types ( and others) I’ve never come across this before.

So, I’ve slipped a shim in between the caliper piston and pad on the inner side and clearance on all pads seems normal.
I’ve got one of those cheap plastic copies of the mityvac suction suction tools so will have a time bleeding this afternoon and see how it goes.
I’ve a feeling that longer term the IRS is out !

Tim

[mgcjag]

Hi Tim...my only thoughts are on the loose pad side the piston and seal is not free moveing in the bore so as you press the pedal it moves out possibly streaching the seal and as you release the pedal it pulles back......maybe try an even looser pad so the piston has to move that bit more just to see if it then pulles back in....very odd...Steve

[MarekH]

On the rear brakes one pad runs true and close to the disc as it should be on the other pad , closest to the diff the pad has a definite clearance to the disc.

Any thoughts, I’m loath to strip the IRS down

Do you have uneven pad wear, diff side to wheel side?

Without any pressure on the pedal the outer pad runs very close to the disc as you would expect whilst the inner pad has the “ rattling good fit”. With the pedal depressed then both pads grip the disc as you would expect and this small gap disappears. Once the pedal is released then the caliper piston retracts and the gap reappears!

In many years of buggering about with e types ( and others) I’ve never come across this before.

What you appear to be telling us is that the rubber seals on the inner faces of the rear disc calipers flex differently to the rubber seals on the wheel sides of the calipers.

The only differentiators here is the heat radiated by the diff (and then reradited by the rotor) and that diff oil, if leaking out of the sides of the diff, will coat the inner rotor faces and lead to possible uneven pad wear, hence my question above.

kind regards
Marek

[tim wood]

Marek, thanks for your comments. I’m not sure my reply will help much !

The pads are relatively new and show clear signs of contact with the discs. They appear to be wearing evenly.

The diff was rebuilt by Alan Slawson, no oil leaks and discs are clean.



Tim

[rfs1957]

If the discs are indeed running true, there’s perhaps another explanation for this.

Shimming the caliper to centre it should have no effect whatsoever on what gaps or play you end up with ; my understanding of the shimming is that it is merely to allow the maximum depth of identical pads to be crammed into the available space.

The slight retraction of the pistons into their bores, once you remove pedal pressure, is provoked simply by the residual grip of the seals on the piston, and the desire of those seals to revert to their relaxed shape.

The essentially square section seal is deformed to trapezoidal, and then reverts to square, pulling the piston back as it does so.

As the pad wears, so the piston progressively overcomes the grip of the seals, but these still do the square-trapedzoidal-square dance during every brake application.

Ii sounds to me as if you have a couple of pistons which might be very dry and “grippy” on their seals, hence the relaxation of the seals is provoking a higher retraction, hence the bigger gap.

Did you grease the pistons and the seals with a suitable lubricant when you assembled them, like the specialised compounds that exist for this very purpose, and might it be worth stripping them to try this ?

[tim wood]

A very good point - thanks! Seems a likely possibility
It all points to a strip down at some point. Can’t say that I’m looking forward to it though.

I think that I will leave as is until the autumn and then pull the calipers. Maybe I’ll update this thread then.

Thanks to all respondents with ideas.

Tim

[MarekH]

So either the rubber seals flex slightly differently in the time it takes to release pressure or there's a bit of rust on the piston which deters it from moving back.

It all points to a strip down at some point.

Why? It's not actually broken in any way and the brakes work....

If you find nothing when you strip it down, then make sure you label everything. Then swap the pistons around and see whether the "fault" follows the pistons... Then strip it down again and swap which sides the seals are in the calipers and recheck. Even then, you may not find the smoking gun.

kind regards
Marek

[tim wood]

Thought that I would close this thread off. Many posters just leave threads hanging.

So I fitted shims to the pads in the rear calipers then in a fit of spending purchased some Fosseways for the front. Very good looking bits of kit but whether they made any difference I don’t know.

What did make a difference was engaging my long suffering wife to assist with the bleeding. I now have a lovely firm pedal. I have pressure bleeding kit, Suction pumps, non return valves but plain old fashioned manual help got it hard !
Mrs W can now add brake bleeding to her skills as a windscreen fitter.

Thanks to all who contributed

Tim.

[DWW]

Thanks for closing the thread, I agree so many left hanging in the technical world. As far as the Fosseways it depends what kit you got, I fitted the four pot with vented disks and felt a significant improvement from the stock kit.

[mgcjag]

Great update Tim... old school bleeding always works..trying to get smart with pressure or vacume always seems to cause issues.....Great that your sorted..Steve

[Gfhug]

Tim, if Mrs W can add windscreen wiper fitting to her skill set the you could make good money out of hiring her out

Geoff